The present invention relates to an electron source having a plurality of electron emitting devices, and an image forming apparatus using the electron source for forming images.
Lately, various efforts have been made in research and development of a thin and large screen display apparatus. The inventor of the present invention has been studying the use of cold cathode as an electron source in a thin and large screen display apparatus.
Conventionally, two types of devices, namely thermionic and cold cathode devices, are known as electron emitting devices. Examples of cold cathode devices are surface-conduction-type emitting devices, field-emission-type devices (to be referred to as FE-type devices hereinafter), and metal/insulator/metal type emission devices (to be referred to as MIM-type devices hereinafter).
A known example of the surface-conduction-type emitting devices is described in, e.g., M. I. Elinson, Radio. Eng. Electron Phys., 10, 1290 (1965) and other examples to be described later.
The surface-conduction-type emitting device utilizes a phenomenon in which electron emission is caused in a small-area thin film formed on a substrate, by providing a current parallel to the film surface. The surface-conduction-type emitting device includes devices using an Au thin film (G. Dittmer, "Thin Solid Films", 9,317 (1972)), an In.sub.2 O.sub.3 /SnO.sub.2 thin film (M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf.", 519 (1975)), and a carbon thin film (Hisashi Araki, et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983)), and the like, in addition to an SnO.sub.2 thin film according to Elinson mentioned above.
FIG. 22 is a plan view of the surface-conduction-type emitting device according to M. Hartwell et al. as a typical example of the structures of these surface-conduction-type emitting devices. Referring to FIG. 22, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped plane pattern, as shown in FIG. 22. An electron emitting portion 3005 is formed by performing an electrification process (referred to as an energization forming process to be described later) with respect to the conductive thin film 3004. Referring to FIG. 22, the spacing L is set to 0.5 to 1 mm, and the width W is set to 0.1 mm. The electron emitting portion 3005 is shown in a rectangular shape at the center of the conductive thin film 3004 for the sake of illustrative convenience, however, this does not exactly show the actual position and shape of the electron emitting portion.
In the above surface-conduction-type emitting device by M. Hartwell et al., typically the electron emitting portion 3005 is formed by performing the electrification process called energization forming process for the conductive thin film 3004 before electron emission. According to the energization forming process, electrification is performed by applying a constant or varying DC voltage which increases at a very slow rate of, e.g., 1 V/min, to both ends of the conductive thin film 3004, so as to partially destroy or deform the conductive thin film 3004 or change the properties of the conductive thin film 3004, thereby forming the electron emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 or part where the properties are changed has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the energization forming process, electron emission occurs near the fissure.
Known examples of the FE-type devices are described in W. P. Dyke and W. W. Dolan, "Field Emission", Advance in Electron Physics, 8,89 (1956) and C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47,5248 (1976).
FIG. 23 is a cross-sectional view of the device according to C. A. Spindt et al. as a typical example of the construction of the FE-type devices. Referring to FIG. 23, reference numeral 3010 denotes a substrate; 3011, an emitter wiring comprising an electrically conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. The device is caused to produce field emission from the tip of the emitter cone 3012 by applying an appropriate voltage across the emitter cone 3012 and gate electrode 3014.
In another example of the construction of an FE-type device, the stacked structure of the kind shown in FIG. 23 is not used. Rather, the emitter and gate electrode are arranged on the substrate in a state substantially parallel to the plane of the substrate.
A known example of the MIM-type is described by C. A. Mead, "Operation of tunnel-emission devices", J. Appl. Phys., 32, 646 (1961). FIG. 24 is a sectional view illustrating a typical example of the construction of the MIM-type device. Referring to FIG. 24, reference numeral 3020 denotes a substrate; 3021, a lower electrode consisting of metal; 3022, a thin insulating layer having a thickness on the order of 100 .OMEGA.; and 3023, an upper electrode consisting of metal and having a thickness on the order of 80 to 300 .OMEGA.. The device is caused to produce field emission from the surface of the upper electrode 3023 by applying an appropriate voltage across the upper electrode 3023 and lower electrode 3021.
Since the above-mentioned cold cathode device makes it possible to obtain electron emission at a lower temperature in comparison with a thermionic cathode device, a heater for applying heat is unnecessary. Accordingly, the structure is simpler than that of the thermionic cathode device and it is possible to fabricate devices that are finer. Further, even though a large number of devices are arranged on a substrate at a high density, problems such as fusing of the substrate do not easily occur. In addition, the cold cathode device differs from the thermionic cathode device in that the latter has a slow response because it is operated by heat produced by a heater. Thus, an advantage of the cold cathode device is the quicker response.
For these reasons, extensive research into applications for cold cathode devices is being carried out.
By way of example, among the various cold cathode devices, the surface-conduction-type emitting device is particularly simple in structure and easy to manufacture and therefore is advantageous in that a large number of devices can be formed over a large area. Accordingly, research has been directed to a method of arraying and driving a large number of the devices, as disclosed in Japanese Patent Application Laid-Open No. 64-31332, filed by the present applicant.
Further, applications of surface-conduction-type emitting devices that have been researched are image forming apparatuses such as an image display apparatus and an image recording apparatus, charged beam sources, and the like.
As for applications to image display apparatus, research has been conducted with regard to such an image display apparatus using, in combination, surface-conduction-type emitting devices and phosphors which emit light by colliding with electrons, as disclosed, for example, in the specifications of U.S. Pat. No. 5,066,883 and Japanese Patent Application Laid-Open (KOKAI) Nos. 2-257551 and 4-28137 filed by the present applicant. The image display apparatus using the combination of the surface-conduction-type emitting devices and phosphors is expected to have characteristics superior to those of the conventional image display apparatus of other types. For example, in comparison with a liquid-crystal display apparatus that has become so popular in recent years, the above-mentioned image display apparatus is superior since it emits its own light and therefore does not require back-lighting. It also has a wider viewing angle.
A method of driving a number of FE-type devices in a row is disclosed, for example, in the specification of U.S. Pat. No. 4,904,895 filed by the present applicant. A flat-type display apparatus reported by R. Meyer et al., for example, is known as an example of an application of an FE-type device to an image display apparatus. [R. Meyer: "Recent Development on Microtips Display at LETI", Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6.about.9, (1991).]
An example in which a number of MIM-type devices are arrayed in a row and applied to an image display apparatus is disclosed in the specification of Japanese Patent Application Laid-Open No. 3-55738 filed by the present applicant.
The present inventor has examined surface-conduction-type emitting devices according to various materials, manufacturing methods, and structures, in addition to the above conventional devices. The present inventor has also studied a multi-electron source in which a large number of surface-conduction-type emitting devices are arranged, and an image display apparatus to which this multi-electron source is applied.
The present inventor has also examined a multi-electron source according to an electric wiring method shown in FIG. 25. More specifically, this multi-electron source is constituted by two-dimensionally arranging a large number of surface-conduction-type emitting devices and wiring these devices in a matrix, as shown in FIG. 25.
Referring to FIG. 25, reference numeral 4001 denotes an electron emitting device; 4002, a row-direction wiring; and 4003, a column-direction wiring. In reality, the row-direction wiring 4002 and the column-direction wiring 4003 include limited electrical resistance; yet, in FIG. 25, they are represented as wiring resistances 4004 and 4005. The wiring shown in FIG. 25 is referred to as simple matrix wiring.
In the multi-electron source in which the surface-conduction-type emitting devices are wired in a simple matrix, appropriate electrical signals are supplied to the row-direction wiring 4002 and the column-direction wiring 4003 to output desired electron beams. For instance, when the surface-conduction-type emitting devices of one arbitrary row in the matrix are to be driven, a selection voltage Vs is applied to the row-direction wiring 4002 of the selected row. Simultaneously, a non-selection voltage Vns is applied to the row-direction wiring 4002 of unselected rows. In synchronization with this operation, a driving voltage Ve for emitting electrons is applied to the column-direction wiring 4003. According to this method, a voltage (Ve-Vs) is applied to the surface-conduction-type emitting devices of the selected row, and a voltage (Ve-Vns) is applied to the surface-conduction-type emitting devices of the unselected rows, assuming that a voltage drop caused by the wiring resistances 4004 and 4005 is negligible. When the voltages Ve, Vs, and Vns are set to appropriate levels, electron beams with a desired intensity are outputted from only the selected row of the surface-conduction-type emitting devices. When different levels of driving voltages Ve are applied to the respective column-direction wiring 4003, electron beams with different intensities are output from the respective devices of the selected row. Since the response rate of the surface-conduction-type emitting device is fast, the period of time over which electron beams are output can also be changed in accordance with the period of time for applying the driving voltage Ve.
Hereinafter, the voltage (Ve-Vs), applied to the device when a row is selected, will be referred to as a device voltage Vf.
As another method of obtaining an electron beam from the multi-electron source in which a plurality of surface-conduction-type emitting devices are wired in a simple matrix, instead of connecting a voltage source for applying a driving voltage Ve with the column-direction wiring, a current source for supplying a driving current may be connected so as to apply a selection voltage Vs to a selected row-direction wiring and apply a non-selection voltage Vns to unselected row-direction wirings. According to this method, because the device has a remarkable threshold characteristic, an electron beam can be outputted only from devices of the selected row. Hereinafter, a current flowing in the electron source will be referred to as a device current If, and a current generated by an emitted electron will be referred to as an emission current Ie.
As described above, the multi-electron source, in which surface-conduction-type emitting devices are wired in a simple matrix, has various application possibilities. For instance, by appropriately applying an electric signal corresponding to image data, the multi-electron source can be used as an electron source of an image display apparatus.
However, in reality, the following problems arise in an image display apparatus employing a multi-electron source in which surface-conduction-type emitting devices are wired in a simple matrix.
More specifically, an image display apparatus constructed, as shown in FIG. 26A, with a multi-electron source panel 2300 in which surface-conduction-type emitting devices are wired in a simple matrix, an X driver 2301 which generates a modulation signal for driving the electron source panel 2300, a Y driver 2302 which generates a scan signal, and a flexible substrate 2304 which connects the electron source panel 2300 with each of the drivers 2301 and 2302, has a transmission line illustrated by the equivalent circuit in FIG. 26B when seen from the modulation signal side (the side of the X driver 2301). More specifically, in FIG. 26B, reference numeral 2310 denotes an equivalent circuit of the flexible substrate 2304; 2311, an equivalent circuit of a connection wiring portion provided between the flexible substrate 2304 and the image display area of the electron source panel 2300 where an image is actually displayed; and 2312, an equivalent circuit of the image display area.
In the foregoing configuration, in a case where each characteristic impedance is different among the image display area of the electron source panel 2300, connection wiring area of the electron source panel 2300, and flexible substrate 2304, reflection or the like occurs due to the unmatched impedance among these areas, and ringing is generated in the modulation signal applied to the column-direction wirings. Due to the ringing, the driving voltage of each device varies, and as a result, luminance fluctuates, making it impossible to attain a display image of desired quality.